U.S. Pat. No. 7,469,092, entitled “Method and Apparatus For The Monitoring And Control Of A Process,” describes a method and apparatus for the monitoring and control of a process using tunable diode laser absorption spectroscopy (TDLAS). Briefly stated, the TDLAS method and apparatus involves directing a beam of light, which may be a multiplexed beam of a number of distinct wavelengths, into a boiler combustion chamber to measure boiler combustion properties such as temperature and the concentration of various combustion species including CO, CO2, O2 and H2O. The technique requires a line of sight through the boiler. In fact, many lines of sight are typically required as it is often desirable to measure combustion properties in multiple boiler locations. Typically a wavelength multiplexed laser beam is transmitted from a pitch optic to a catch optic on the opposite side of the boiler. Certain applications require up to 15 measurement paths, thus requiring 15 pitch/catch optic pairs and 30 boiler penetrations.
Typical coal-fire boilers comprise walls made of a series of parallel steam tubes spaced by a metal membrane. The steam tubes are typically about 2 inches (5.08 cm) in diameter and occur on about 2.5 inch (6.35 cm) centers. The metal membrane between the tubes is typically about 0.5 inch (1.27 cm) wide and 0.375 inch (0.9525 cm) thick. To gain optical access for measurements using a wavelength-multiplexed laser beam optical access must be provided through the wall of the boiler. Known TDLAS apparatus require an approximately 2 inch (5.08 cm) diameter hole in the boiler wall to provide adequate optical access.
FIG. 1 illustrates the current state of the art for providing optical access to a boiler interior. Referring to FIG. 1, the boiler wall 10 comprises a series of parallel steam tubes 12 separated by a metal membrane 14. To provide the required 2″ hole for optical access, the tubes must be rerouted using tube bends as illustrated in FIG. 1. Once completed, the use of tube bends to provide the optical access works well. However, it is difficult and expensive to provide the required number of tube bends for satisfactory combustion monitoring. The problem stems primarily from the fact that in order to install even a single tube bend, the boiler must be shut down for a significant period of time. As a result, the tube bends and thus the TDLAS monitor can only be installed during a long planned outage. Planned outages occur only every one or two years. Thus, unfortunate timing may result in having to wait up to two years before a particular power plant will be in a position to purchase and install a TDLAS monitor. Thus, an apparatus for monitoring combustion properties within a boiler that eliminates the need for tube bends is highly desirable.
The present invention is directed toward overcoming one or more of the problems discussed above.